A gas laser oscillator such as a carbon dioxide gas laser oscillator can provide high efficiency and high output power, so that it has widely been employed in a laser beam machining field.
In the case of the aforesaid carbon dioxide gas laser oscillator, a CO.sub.2 molecule which is used as a laser medium is excited by a discharge generated in the resonator. However, an electric power supplied for the discharge is subject to limitation due to a durability of members constituting an excitation section and a rise of gas temperature caused by an excitation. Further, the aforesaid laser needs to enlarge an output beam diameter due to limitation of light-proof strength of an output mirror. For this reason, it is necessary to make an optical path of the resonator long in order to obtain high output power; as a consequence, the laser system becomes too large as a whole. Thus, in order to avoid enlarging the configuration of the system, most of the laser oscillators are constituted such that the optical path of the resonator is multitudinously folded back so that an entire length of the laser oscillator becomes short.
FIG. 5 is a view schematically showing a configuration of a conventional folded-type resonator. In the figure, a resonator 20 comprises an output mirror 1, a rear mirror 2, excitation sections 3a and 3b, and reflecting mirrors 14a and 14b. All optical axes in the resonator 20 are adjusted so as to be in the identical horizontal plane. In the resonator 20 having such optical axes, the laser beam emitted from the output mirror 1 is polarized, in the vertical direction at an angle of 90.degree. to the horizontal plane, as shown by an arrow 21.
When a laser beam is used for cutting metals or the like, it is desirable that a circularly polarized laser beam be irradiated to a workpiece in order to obtain high machining performance rather than a linearly polarized laser beam. For this reason, a linearly polarized laser beam in the vertical or horizontal direction is generally transformed to a circularly polarized laser beam by means of a mechanism as shown in FIG. 6.
FIG. 6 is a view showing a conventional optical system for transforming a linearly polarized laser beam to a circularly polarized laser beam. A laser beam emitted from the resonator 20 is polarized linearly in the vertical direction as shown by an arrow 21. In order to transform the aforesaid laser beam into a circularly polarized laser beam, a polarization mirror 5 for polarizing the laser beam linearly at an angle of 45.degree. and a 1/4 wave-length mirror 6 are required. The laser beam, which is transformed into a circularly polarized laser beam by the polarization mirror 5 and 1/4 wave-length mirror 6, is further directed in the vertical direction by a mirror 7, and is converged by a condenser lens 8, thus being irradiated to a workpiece 9. The above explanation refers to a case of transforming a linearly polarized laser beam in the vertical direction into a circularly polarized laser beam. Likewise, a linearly polarized laser beam in the horizontal direction is transformed into a circularly polarized laser beam by the same optical system as described above.
As seen from FIG. 6, the conventional laser oscillator requires at least the polarization mirror 5 and 1/4 wave-length mirror 6 in order to transform a linearly polarized laser beam in the vertical or horizontal direction into a circularly polarized laser beam. In addition, the conventional laser oscillator requires the mirror 7 for changing the direction of a laser beam into the vertically downward direction because a workpiece is usually placed on the horizontal plane. In other words, three external mirrors are required. For these additional mirrors, the cost of the laser oscillator, as a whole, becomes high.
These external mirrors are frequently contaminated with dust and the like, in contrast to mirrors included in the resonator. For this reason, the maintenance operations such as disassembling and cleansing are required. In addition, when assembling them after cleaning, it is necessary to make a complicated adjustment of the external optical system.